1. Field of the Invention
This invention relates to a reheat combustion system for a gas turbine. In particular, the invention relates to such a system comprising acoustic damping.
In modern industrial gas turbines operating with pre-mix combustion flames, it is important to suppress pressure pulsations in order to maintain the quality of the combustion process and preserve structural integrity of the turbine. To date, acoustic damping techniques have been employed in order to dissipate acoustic power and thereby reduce the pressure pulsations.
2. Brief Description of Related Art
In conventional gas turbines (having only one combustion zone) it is known to damp low frequency pulsations using Helmholtz resonators. The simplest design for a Helmholtz resonator comprises a cavity, with a neck through which the fluid inside the resonator communicates with an enclosure that the resonator is applied to. At its resonance frequency, the Helmholtz resonator is able to produce a small acoustic pressure on the mouth of its neck. When the resonance frequency of the resonator coincides with an eigenfrequency of the enclosure with a mode having a high-pressure value where the resonator neck is located, then the resonator is able to damp the acoustic mode.
The advantage of a Helmholtz resonator is that the area of the neck mouth may be considerably smaller than the boundary of the enclosure. On the other hand, Helmholtz resonators may damp only single modes, with a damping efficiency proportional to the volume of the resonator cavity. Consequently, Helmholtz resonators are normally confined for use in the low frequency range, where the frequency shift between acoustic modes is relatively large (i.e. pressure peaks are well separated) and the resonator volume is also relatively large.
As an alternative to Helmholtz resonators, it is known to use quarter wavelength dampers. In such dampers, the cavity and neck of a Helmholtz resonator are replaced by a single tube.
In a gas turbine comprising a reheat combustion system, a secondary combustion zone is realised by injecting fuel into a high velocity gas stream formed by the products of the primary combustion zone. Consequently, combustion occurs without the need for flame stabilisation and high-frequency pulsations are generated. In such a case, classical Helmholtz resonators are not optimal for the frequency range in question.
To damp high-frequency noise generated in rocket engines and aircraft engines, acoustic liners are usually employed. A liner typically consists of a perforated screen which lines the engine ducts (for example the fan ducts of a turbo fan engine). An inperforated screen is provided behind the perforated screen and a honeycomb core is generally located between the two screens.
The goal of the liner is to provide a wall which does not fully reflect acoustically and is able to damp pulsations across a broad range of frequencies. The acoustic behaviour of the liner is defined by means of its impedance Z=R+iX. That is to say, the ratio between acoustic pressure and velocity of the fluid normal to the wall, both being defined in the frequency domain. The real part R of the impedance is the resistance, determined by dissipative processes occurring in the voids of the liner. The main dissipative effect is the conversion of acoustic energy into a shedding of vorticity, generated at the rims of the perforations in the screen, convected downstream and finally dissipated into heat by turbulence. The imaginary part X of the impedance is the reactance, which represents the inertia of the fluid fluctuating in the perforations and in the cavity between the two screens under the effect of the acoustic field.
To damp high order modes (i.e. for high-frequency applications), the liners are typically designed to have a resistance R close to ρc (wherein ρ is the fluid density and c the speed of sound in the fluid) and reactance X close to 0. It should be understood that the conditions R=ρc and X=0 correspond to the anechoic condition (that is to say the full absorption of acoustic energy of a normally incident plane wave).
Converse to for the situation with a Helmholtz damper, the efficiency of the liner is strongly related to the portion of the surface that the liner covers. Consequently, different liner designs have been proposed, in which the damped frequency band was extended by use of a multi-layer liners or by a non uniform distribution of honeycomb cells between the two screens. However, the walls of the burner and combustion chamber must be cooled by means of cold air coming from the compressor and the acoustic liners do not readily facilitate this.